Method and apparatus for eddy current testing by at least two different frequency signals

Abstract

During so-called eddy current testing by way of several frequencies, it is often difficult efficiently to separate and detect superficially located defects from the disturbances caused by a varying distance from the transducer to the object under test. This is due to the fact that the fault direction and the lift-off direction normally differ from each other to too small an extent and the fact that the signal processing, which is necessary for the suppression of the lift-off dependence, also suppresses the fault signal. This problem can be eliminated completely or partially by means of the present invention, which comprises selecting the used frequencies in such a way that, in relation to the depths of cracks occurring, they fulfill certain conditions, one of which being that at least one frequency is relatively insensitive to the defect in question. This effect is achieved by selecting such a high frequency that its depth of current penetration is limited seen in relation to the depth of the defect.

I claim: 
     
       1. A method for testing electrically conducting test objects for defects therein, comprising: exciting a transducer with signals of different frequencies to induce eddy currents in said test object;   deriving at least two test signals from said transducer such that the quotient of said at least two test signals is substantially constant as a function of the distance of said transducer from the surface of said test object;   selecting one transducer frequency excitation to produce a current penetration depth less than a specified penetration depth;   selecting the lowest transducer excitation frequency to provide a current penetration depth at least three times greater than the depth of a second specified detection depth; and   detecting a defect by forming a quotient between said at least two test signals, thereby determining such defects with small angle relationships between the fault vector and the lift-off vector.   
     
     
       2. Apparatus for testing electrically conducting test objects for defects therein, comprising: at least one transducer excited by signals of different frequency to induce eddy currents in said test object;   means for detecting said eddy currents to produce at least two signals having at least partially different frequency content, the quotient of said at least two signals having a substantially constant relationship with respect to one another as a function of the distance of said at least one transducer from the surface of said test object with no detection of a defect therein, and the quotient of said at least two signals having a substantially different relationship in the presence of a defect;   at least one of said signals of different frequency produces an eddy current penetration depth smaller than the depth of specified defects;   the lowest signal frequency producing an eddy current penetration depth at least three times greater than another specified depth; and   means for determining a defect by a predetermined difference in the quotient of said at least two signals, thereby determining such defects with small angle relationships between the fault vector and the lift-off vector.   
     
     
       3. Apparatus as claimed in claim 2 wherein said test object has a temperature greater than 700 degrees centigrade and that at least one frequency is greater than 100 KHz. 
     
     
       4. Apparatus as claimed in claim 2 wherein said means for detecting are positioned in close proximity to said at least one transducer. 
     
     
       5. Apparatus as claimed in claim 2 further comprising an insulator containing aluminum oxide is positioned between said at least one transducer and said test object.